No, 1 chastity is not the same as 2 fireballs. In war you don't just lose peasants, you lose your army as well. When you get hit(not chained, just once is enough), you might overpop and lose peasants that you cannot replace(which easily amounts to a few thousands of them) even with incoming. Having chastity on is like capping a provinces' ability to grow or draft completely even with incoming lands.
What most of you defending it don't see is that birth rates is not the sole problem, birthrates cap leads to MANY other issues. That's what makes chastity OP. I wonder how many of you actually been in chastity coverage(or if the enemy is doing it right at all), because from what I observe, all you people ever think chastity is about is birth rates that's all.